In an integrated circuit, the transistors are produced in and on an active semiconductor region, for example of silicon, surrounded by an electrically insulating region, for example a trench filled, for example, with silicon dioxide.
Production of an MOS transistor within an insulating region inherently leads to an active region being obtained which is compressively stressed owing to the presence of the insulating region at its periphery. Furthermore, although a compressively stressed active region promotes the performance of a PMOS transistor, it conversely causes a degradation of the performance of an NMOS transistor, particularly in terms of mobility of the carriers.
Furthermore, the production of high-speed transistors requires small channel lengths and widths, and structures generally produced have a high density, which leads to active region dimensions which are very small or even minimal for the technology in question.
It is therefore extremely difficult or even impossible to increase the dimensions of the active regions of the NMOS transistors with a view to relaxing their compressive stresses, in view of the desired density of the structures produced.